news digest ♦ Power Electronics
Source: EPC Corporation The implication is that a point- of-load (POL) converter using GaN can convert from 48V to 1V in a single stage, while an equivalent silicon converter would typically require one stage to convert to 12V and a second stage to convert to 1V. At higher drain-source voltages, the superiority of GaN becomes even more apparent, enabling entirely new architectures in power management, according to proponents such as EPC and International Rectifier. In the past several years, there has been a concerted effort to develop enhancement-mode (normally off) GaN transistors, especially for power management applications. These enhancement-mode devices are attractive for these applications because they operate in a similar fashion to the incumbent MOSFET technology, but have much better performance characteristics.
A number of Japanese companies, including Sanken Electric, a collaboration between Fuji and Furukawa Electric, Panasonic and NEC, have all invested in the development of E-mode GaN HEMTs. In March 2010, Efficient Power Conversion (EPC) Corporation introduced a number of E-mode GaN-on-silicon power management devices, branded as eGaN, which they continue to refine and improve. These E-mode devices have different driver requirements than their depletion-mode GaN and silicon MOSFET counterparts. Companies like National Semiconductor and Texas Instruments have developed lines of compatible drivers for E-mode GaN devices.
These driver efforts are likely to speed adoption of E-mode GaN devices by making the final package of driver and transistor as easy to use as MOSFET devices. For all the inherent material and performance advantages, developers of GaN transistor technology continue to address a number of challenges. The lack of suitable native GaN substrates complicates production of GaN devices because it is very difficult to grow lattice-matched, defect-free epilayers analogous to the processes used for GaAs or silicon transistor fabrication. There is substantial activity using a hydride vapor phase epitaxy (HVPE) method to produce thick layers that can serve as quasi-bulk substrates. Shortfalls with this technique have given rise to development of ammonothermal growth techniques.
In August 2012, Soraa, a developer of GaN-on-GaN solid-state lighting technology, was selected by Advanced Research Projects Agency-Energy (ARPA-E) to lead a project on the development of bulk GaN substrates. The attraction of native GaN substrates lies in an anticipated performance improvement that may result in significant energy savings for LEDs. The LED market can provide the high-volume pull to develop a technology that can benefit other market segments with the availability of native GaN substrates. Even with a production ready process, GaN substrates are not expected to compete
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with silicon substrates purely on wafer cost. However, proponents of native GaN substrates point out that a simplified process will result in cost savings that may make the product cost more manageable. In the absence of production-scale, single-crystal GaN wafers, manufacturers must instead use foreign host materials such as sapphire, SiC or silicon. To date, semi-insulating SiC has been the material of choice for microelectronic components, thanks to its relatively close lattice match to GaN and its excellent thermal conductivity properties.
However, the quality of epitaxial layers depends on both the lattice-match and the underlying substrate quality. Historically, suppliers of semi-insulating SiC have struggled to produce material with defect density levels comparable to substrates such as silicon or GaAs. More recently, improvements to semi-insulating SiC material quality, along with the availability of larger substrates, have made SiC a more viable economic choice for GaN growth. This has helped to improve the reliability of GaN microelectronic devices.
However, SiC remains a difficult and expensive material to produce and it provides cost challenges for cost- sensitive, high-volume applications. In recent years, high- resistivity (HR) silicon has become a viable alternative to SiC in certain applications. Although it does not have as close a lattice match with GaN and possesses poorer thermal properties, silicon can offer a lower- cost path for some applications. GaN-on-Si also shows potential to transition to high-volume manufacturing processes because it is amenable to existing CMOS (Complementary Metal-Oxide-Semiconductor) semiconductor fabrication technology using commercially available, large diameter silicon wafers. There is a lot of development work in this area with manufacturers like AZZURRO Semiconductors developing 150mm GaN-on- silicon wafers with a roadmap to 300mm diameter wafers in the future.
Future The size of the power electronics component market and the perceived advantages of wide bandgap technologies are leading to significant process and product development efforts. The US government realizes the importance of improvements in the efficiency and reliability of the electrical grid network and the US Department of Energy’s Office of Electricity Delivery and Energy Reliability (OE) has requested slightly more than $169 million in the FY2014 US budget to address issues and developments in this area. The budget request is up nearly 25% from 2012 spending as the OE recognizes the importance of the topic and realizes that industry will need some help to enable significant advances. In April 2011, the OE released their “Power Electronics Research and Development Program Plan” that details the vision, activities, challenges, needs and partnership strategies for this market segment. The activities focus clearly on wide bandgap materials, with a heavy emphasis on
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